The thermal equation of state (EoS) of a natural schorl has been determined at high temperatures up to 673 K and high pressures up to 15.5 GPa using in situ synchrotron X-ray diffraction combined with a diamond-anvil cell. The pressure-volume (P-V) data were fitted to a third-order Birch-Murnaghan EoS with V0 = 1581.45 ± 0.25 Å3, K0 = 111.6 ± 0.9 GPa, and K0′ = 4.4 ± 0.2; additionally, when K0′ was fixed at a value of 4, V0 = 1581.04 ± 0.20 Å3, and K0 = 113.6 ± 0.3 GPa. The V0 (1581.45 ± 0.25 Å3) obtained by the third-order Birch-Murnaghan EoS agrees well with the V0 (1581.45 ± 0.05 Å3) measured at ambient conditions. Furthermore, the axial compression data of schorl at room temperature were fitted to a “linearized” third-order Birch-Murnaghan EoS, and the obtained axial moduli for the a- and c-axes are Ka = 621 ± 9 GPa and Kc = 174 ± 2 GPa, respectively. Consequently, the axial compressibilities are βa = 1.61 × 10–3 GPa–1 and βc = 5.75 × 10–3 GPa–1 with an anisotropic ratio of βa:βc = 0.28:1.00, indicating axial compression anisotropy. In addition, the compositional effect on the axial compressibilities of tourmalines was discussed. Fitting our pressure-volume-temperature (P-V-T) data to a high-temperature third-order Birch-Murnaghan EoS yielded the following thermal EoS parameters: V0 = 1581.2 ± 0.2 Å3, K0 = 110.5 ± 0.6 GPa, K0′ = 4.6 ± 0.2, (∂KT/∂T)P = –0.012 ± 0.003 GPa K–1 and αV0 = (2.4 ± 0.2) × 10–5 K–1. These parameters were compared with those of previous studies on other tourmalines, and the potential factors influencing the thermal EoS parameters of tourmalines were further discussed.

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